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J. Phys. Chem. C 2008, 112, 18340–18349
Gold Nanospheres and Nanonecklaces Generated by Laser Ablation in Supercritical Fluid Ken-ichi Saitow,*,†,‡,§ Tomoharu Yamamura,§ and Takamasa Minami§ Natural Science Center for Basic Research and DeVelopment (N-BARD), and Department of Chemistry, Graduate School of Science, Hiroshima UniVersity, 1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan, and PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ReceiVed: July 7, 2008; ReVised Manuscript ReceiVed: September 5, 2008
Nanosecond pulsed laser ablation of gold with an excitation wavelength of 532 nm was conducted in supercritical CO2 to generate gold nanoparticles, which were then investigated by scanning electron microscopy and small-angle X-ray scattering, and their extinction spectra and simulated extinction spectra were studied. Both the morphology and amount of gold nanoparticles changed significantly with changes in the density of supercritical CO2 during laser ablation. In a gaslike density, a network structure consisting of nanonecklaces was the major product, whereas in a liquidlike density, large nanospheres with an average diameter (〈D〉) of 500 nm were produced. After absorption of multiphoton of excitation light, the gold nanonecklaces and large nanospheres were generated by the fragmentation and solidification, respectively, of liquid gold droplets with 〈D〉 ) 500 nm. The amount of both products changed according to the branching ratio, which determined whether the liquid gold droplets followed the fragmentation or solidification channel. The local structure of supercritical CO2 in the vicinity of the gold nanoparticles determined the preferred reaction channel. A significant change in the branching ratio occurred near the density Fr ) 0.7, where both the enhancement of the local density of supercritical CO2 and the degree of solvation of fluid molecules around the gold nanoparticles reached a maximum. To the best of our knowledge, this is the first study to observe the density dependence of morphological changes in gold nanoparticles fabricated by laser ablation in a supercritical fluid and the local structure of the supercritical fluid that determines the morphology and amount of nanoparticles. Introduction Noble metal nanoparticles are fascinating materials with novel properties applicable to both natural and applied sciences. For example, gold nanoparticles and nanoclusters1 have been used as materials for surface-enhanced Raman scattering,2 the building blocks of nanostructures,3 catalysts for CO oxidation,4 ultrasensitive bio- and medical sensors,5 and cancer diagnostics and therapies.6 Recently, gold nanoparticles of well-defined size and shape have been synthesized by chemical reduction of gold ions in solution. This chemical method, however, can cause contamination from excess reagents such as residual reductants, surfactants, or ions and can lead to functionalization of gold nanoparticles with many kinds of molecules. As an alternative method, laser ablation has been used for the synthesis of gold nanoparticles over the last few decades. During the past decade, the number of studies on gold exposed to the intensive electric fields of lasers has significantly increased.7-44 Our understanding of pulsed laser ablation of gold has improved both experimentally and theoretically. In these studies, nano-,8-10,21-23,28-36 pico-,11-13,39 and femtosecond7,14-20,24-27,37,38 laser pulses, ranging from the ultraviolet to near-infrared regions, with fluence up to a few tens of joules per square centimeter, have been used as light sources. The gold target irradiated with a laser pulse can be in the form of * To whom correspondence should be addressed. Telephone and fax: +81-82-424-7487. E-mail:
[email protected]. † N-BARD, Hiroshima University. ‡ PRESTO, Japan Science and Technology Agency. § Department of Chemistry, Graduate School of Science, Hiroshima University.
a plate,9,10,19,20,23-28,30-32 particle,7,8,11-18,29,34-39 flake,21 or Au3+ in a HAuCl4 solution,22 and the medium surrounding the target can consist of vacuum,23-25 water,20,21,27 an aqueous solution,7-9,11-19,26-29 an organic liquid,10,30-32,34 or an ionic liquid.33 A large number of studies on various systems have been conducted because of (i) the usefulness of pulsed laser ablation for quickly obtaining nanostructured materials, (ii) its negligible amounts of contamination in comparison to chemical methods, and (iii) the progress of laser technology in producing high-fluence pulsed lasers. Studies on pulsed laser ablation have produced fruitful results on the static and dynamic aspects of gold nanoparticles. In terms of the static structure, observations with electron and probe microscopes and spectral measurement of the surface plasmon band help monitor the size and morphology of the generated gold nanoparticles while varying the fluence,11,13,14,21,30 pulse width,14,37,38 solvents,30-34 and concentration of the surfactant.8,9,13,27-29 To understand the dynamics, ultrafast absorption spectroscopy,7,14-16 X-ray diffraction,17,18 and absorption39 in the pico- to nanosecond region, absorption,35 emission and imaging23,24 ranging from the nano- to microsecond region, and detailed molecular dynamics simulations40,41 have been used to clarify the time evolution of gold immediately after laser irradiation. From the results, photofragmentation to smaller particles,7,8,11,13,14,18,20,21,29,34,37-39 oscillation due to coherent volume change,15,16 time-dependent electronic and lattice temperatures,15-18,40,41 thermal melting,8,11,12,17,18,37-41 boiling and evaporation,7,11,37-39 plasma formation,23-25,27 and Coulomb explosion20,35,36 have been described to characterize the dynamics when gold is exposed to intense laser radiation. Note that most of the static and dynamic features that have been reported involve a
10.1021/jp805978g CCC: $40.75 2008 American Chemical Society Published on Web 11/01/2008
Gold Nanospheres and Nanonecklaces change in the medium around gold. As a result, systematic research carried out by changing the dielectric medium is obviously important for understanding the mechanism of laser ablation as well as the generation of gold nanoparticles. However, such comprehensive systematic research has not yet been conducted, as few suitable systems are available. From the viewpoint of systematic research, supercritical fluid is an attractive medium because, given the absence of the liquid-vapor phase transition, changing the pressure can change its density continuously by a factor of a few hundreds.45 In particular, the local structure of a supercritical fluid (i.e., the dielectric medium directly surrounding the vicinity of a molecule) changes from a gaslike to a liquidlike structure with the extension of the liquid-vapor coexistence curve beyond the critical point on the pressure-temperature (P-T) phase diagram.46-49 In addition, physical properties such as thermal conductivity, heat capacity, dielectric constant, and diffusion constant are easily controlled by changing the pressure.45 Thus, the medium surrounding gold, which is irradiated with an intense laser, can be adjusted continuously by manipulating the supercritical fluid. In a previous study, we conducted laser ablation in a supercritical fluid for the first time.50 We irradiated a silicon single crystal immersed in supercritical CO2 with a nanosecond pulsed laser. Changing the pressure or density during laser ablation generated silicon nanoclusters with a variety of electronic structures. In the present study, we performed laser ablation of gold in a supercritical fluid. The density of supercritical CO2 was varied during the laser ablation, and gold nanoparticles were generated. We investigated the size, morphology, and number of generated gold nanoparticles by scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), extinction spectra, and simulation of the obtained spectra. The results showed that a gaslike structure of supercritical CO2 produces a network structure consisting of strings of gold nanospheres, whereas a liquidlike structure produces large 500-nm-diameter gold nanospheres. The former and latter products were formed by the fragmentation of liquid gold droplets and their solidification, respectively. The branching ratio of the products changed significantly near the reduced density Fr ) F/Fc ) 0.7, where both the local density enhancement of fluid molecules and the degree of solvation of gold nanoparticles reaches a maximum. Experiments conducted with varying laser power determined that gold nanoparticles are generated in supercritical CO2 by the absorption of five to six 532-nm-wavelength photons. We consider this multiphoton process to be the driving force behind fragmentation of the liquid gold droplets. Experimental Methods We developed an instrument to fabricate nanoparticles50 consisting of several components such as a high-pressure cell, a Q-switched frequency-doubled Nd:YAG laser (Litron Optical, LPU-4000), and an in situ absorption spectrometer. The highpressure cell was made of stainless steel (SUS316). The cell windows were made of sapphire and sealed with Teflon gaskets. The fluid pressure was increased to 20 MPa with an HPLC pump (Nihonseimitsu Kagaku, NP-S-323). The temperature was controlled by a set of heaters, a proportional integral derivative controller (Chino, DB-1000), and a thermocouple. The Nd:YAG laser served as a light source for pulsed laser ablation and was operated with an excitation wavelength of 532 nm, energy of 19 mJ/pulse, repetition rate of 20 Hz, and fluence of 0.8 J/cm2. The pulse width was 9 ns when measured with a fast PIN photodiode and a digital oscilloscope. A gold plate
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Figure 1. Laser ablation of gold performed at the displayed thermodynamic states of supercritical CO2. Measurements of SEM and extinction spectra were conducted at the thermodynamic states of blue circles and red squares, respectively. (a) Pressure-temperature phase diagram of CO2 in the vicinity of the liquid-vapor critical point (CP). Solid line is the liquid-vapor coexistence curve. (b) Pressure-density phase diagram. Solid curve is obtained from the empirical equation of state of CO2.
immersed in supercritical CO2 was irradiated with the laser for 5 min at an isothermal condition of T ) 310.2 K. Thermodynamic states of supercritical CO2 are plotted on the P-T phase diagram in Figure 1a together with an isotherm at a reduced temperature Tr ) T/Tc ) 1.02. The corresponding pressure vs density curve, shown in Figure 1b, is calculated from the empirical equation of state of supercritical CO2, using measured values of P and T.51 The absorption spectrometer was designed to measure the spectrum in situ with the supercritical fluid under high pressure. A halogen lamp (Ocean Optics, LS-1) served as the light source, and a CCD camera equipped with a monochromator (Ocean Optics, USB4000) served as the detector. The light was introduced into an optical fiber equipped with a collimation lens, and the collimated light was passed through the sample cell. The output light from the cell was focused into another optical fiber and detected with the CCD camera. Using a pair of optical fibers provided high flexibility for in situ measurement and also minimized stray light. The detection limit of absorbance was confirmed to be 10-3 under high pressure conditions. The absorption spectrum was calculated from the equation log(I0(ω)/ I(ω)), where I0(ω) and I(ω) are the transmission spectra before and after laser irradiation, respectively. We were able to extract the spectral change before and after ablation to obtain the absorbance change. The gold nanoparticles generated at each density were deposited as sediments on a carbon disk in supercritical CO2. After sedimentation, the carbon disk was removed from the cell and examined by SEM (Hitachi S-3400N) and field emission SEM (FE-SEM, Hitachi S-5200). The sedimentation time was estimated as52
18342 J. Phys. Chem. C, Vol. 112, No. 47, 2008 2 dz (F - F0)D g νt ) ) dt 18η
Saitow et al.
(1)
where Vt is the sedimentation velocity, z is the depth, t is the time, F is the particle density, F0 is the fluid density, D is the particle diameter, g is the gravitational constant, and η is the viscosity. In the present study, the sedimentation time was adjusted to the amount of time required for a 100-nm-diameter sphere to sink to a depth of 1 cm. By applying this sedimentation condition at each density, SEM samples were prepared to correctly evaluate the amount of gold nanoparticles generated at each density. To investigate the size of the gold nanoparticles, we conducted SAXS measurements with a commercial X-ray diffraction system (Rigaku, RINT-TTR III). The gold nanoparticles generated were deposited in supercritical CO2 as sediments on a Kapton film, which we placed in a sample holder that was normal to the incident X-rays. The X-rays scattered from the sample were detected with a CCD camera, and the scattering intensity was recorded as a function of the scattering angle. Using the same experimental configuration, we recorded X-rays scattered from a blank Kapton film to compensate for background signals. The chemical purities of CO2 (Taiyo Nippon Sanso Co. Ltd.) and the gold plate (Tanaka Co.) were both commercially guaranteed to be 99.99%. The critical constants of CO2 are Tc ) 304.13 K, Pc ) 7.377 MPa, and Fc ) 0.465 gcm-3.51 We represent the density of CO2 as the reduced density Fr ) F/Fc, as shown in the top axis of Figure 1a.
respectively. These data show that the morphology of the products at low and high densities differs. The morphology at low density resembles a network, whereas that at high density is a spherelike structure. Figure 2c,d shows magnified images measured with FE-SEM. The network structure consists of a necklace of nanospheres with an average diameter of approximately 30 nm. As the density increased, large nanospheres with diameters of about 500 nm were generated. It is known that gold spheres with diameters larger than 100 nm are difficult to synthesize by the conventional method (i.e., by reducing HAuCl4 in solution). However, by laser ablation in a supercritical fluid, huge spheres are obtained easily within a few minutes. Figure 2 also shows that the nanonecklace has a very long structure, with lengths on the order of a few tens of micrometers. To the best of our knowledge, a nanonecklace of this length has not been reported previously, although similar necklace structures of shorter lengths (i.e., a few hundred nanometers) have been observed.19,20,53 To evaluate the size distribution, we performed SAXS experiments. Figure 3a shows the typical profile of the scattered X-rays as a function of the scattering vector. The obtained profile was analyzed using the scattering curve of a sphere measured in the transmission configuration, which incorporates the Γ function to obtain the size distribution function, as shown below.54
I(θ) ∝ |F(q, R0, M)|2 )
M
( )
∫0M e-MR⁄R R-1+M 0
R0 3 × R
|reFf(q)Ω(q, R)|2 dR (2)
Results It was confirmed that the gold nanoparticles were generated by laser ablation of the gold plate for all densities of supercritical CO2 studied. Typical SEM images of the generated gold nanoparticles at low and high densities are shown in Figure 2a,b,
( )
1 M Γ(M) R0
Ω(q, R) )
4πR3 [sin(qR) - (qR)cos(qR)] (qR)3
(3)
where I(θ) is the scattering intensity as a function of the scattering angle θ, qis the scattering vector, R0 is the averaged
Figure 2. SEM images of gold nanoparticles generated by laser ablation in supercritical CO2. (a) Nanoparticles generated by laser ablation at Fr ) 0.2 and 4.29 MPa. (b) Nanoparticles generated by laser ablation at Fr ) 1.7 and 14.5 MPa. (c) Magnified image of (a) measured by FE-SEM. (d) Magnified image of (b) measured by FE-SEM.
Gold Nanospheres and Nanonecklaces
Figure 3. Data obtained from small-angle X-ray scattering of gold nanoparticles. (a) Scattering profile of gold nanoparticles generated at the density Fr ) 0.2 as a function of the scattering vector q. Red curve is the fitted function indicating the size distribution of gold nanoparticles (see ref 54 for analysis method). (b) Size distribution of nanospheres consisting of gold nanonecklaces generated at the density at Fr ) 0.2. (c) Size distribution of nanospheres consisting of gold nanonecklaces generated at the density Fr ) 1.1.
radius of spherical particles, M is the shape parameter, Γ is the gamma function indicating the size distribution, R is the radius of a spherical particle, re is the radius of an electron, F is the electron density, f(q) is the average atomic scattering factor at q, and Ω(q,R) is the form factor of the spherical particle with radius R at q. q ) 4π sin θ/λ, where λ is the wavelength of the X-rays (Cu KR, λ ) 1.5 Å). The black and red curves in Figure 3a represent the experimental data and the fitting function, respectively. By using the best-fitted data, we obtained the distribution function characterizing the diameter of gold nanospheres. As shown in Figure 3b,c, the size distribution has two distinct features. First, the peak position at 30 nm is in good agreement with the diameter of the nanospheres in the network observed in SEM measurements. The average diameter 〈D〉 of the gold nanospheres in the necklaces was estimated to be 40 ( 10 nm. Second, the distribution functions for low and high densities are similar, which indicates that the size of nanospheres in the necklaces does not depend on the density. To obtain a larger size distribution, we analyzed SEM images for each density because the data from SAXS measurements are difficult to analyze for particles larger than 200 nm. The size of large nanoparticles in SEM images was analyzed with a software package (Mountech, Mac-View 4). Figure 4 shows histograms of the size distribution of large nanospheres for three densities. The principal size is independent of fluid density. Thus, we estimated the average diameter of the large nanospheres as 〈D〉 ) 520 ( 130 nm. According to the results of
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Figure 4. Histograms of size distributions of large gold nanospheres at three densities. The distributions were obtained from analysis of SEM images. Left and right axes represent the number of large nanospheres and the accumulated percentage of that number, respectively.
SAXS and SEM analysis for the size distributions, we concluded that nanosphere size does not depend on the changes in the density of supercritical CO2. Next, we investigated the amount of gold nanoparticles as a function of density. As shown in Figure 2a,b, as the density increases, the number of nanonecklaces decreases whereas that of large nanospheres increases. To evaluate these density dependencies, the amounts of nanonecklaces and large nanospheres were calculated from the area of the network of nanonecklaces in the SEM image and the number of nanospheres with an average diameter of 520 nm, respectively. That is, utilizing the software package, we estimated the amounts of gold nanonecklaces from the white areas of the gold nanonecklaces by eliminating the white areas of the large gold nanoparticles from the black background of the carbon disk in the SEM images. We obtained the number of large nanospheres by counting the number of gold nanospheres in the SEM images. These results display significant density dependence (Figure 5). The amount of nanonecklaces decreases as the density increases, reaching a constant value when the density increases above Fr ) 0.7. On the other hand, the amount of large nanospheres increases with density, reaching a constant value near the same density. Note that the decreasing profile of the network shows an inverse relation to the increasing profile of the large nanospheres. To confirm the generation processes of gold nanonetworks and large nanospheres, both quantities were measured at a constant density of Fr ) 0.2 by varying the number of laser shot. Figure 6a shows the quantities of the large nanospheres and the network, which are shown on left and right axes, respectively. The large nanosphere is rapidly generated by the laser irradiation, and the network structure appears successively.
18344 J. Phys. Chem. C, Vol. 112, No. 47, 2008
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Figure 5. Density dependences of the quantities of large nanospheres and nanonecklaces. The large nanospheres and nanonecklaces were quantified by counting the number of nanospheres and by estimating the area occupied by the necklaces in each SEM image, respectively. The former and latter quantities are represented by red circles on the left axis and blue circles on right axis, respectively. The bottom and top axes are the reduced density and pressure of supercritical CO2, respectively, during laser ablation.
Figure 7. Extinction spectra of gold nanoparticles measured in situ.
Figure 6. Quantities of large nanospheres and nanonecklaces by varying the laser pulse. (a) Quantities as functions of laser pulses. (b) Quantities divided by the number of laser pulses. The quantities of large nanospheres and nanonecklaces are represented by red circles on the left axis and blue circles on right axis, respectively.
Figure 6b displays the quantities divided by the number of laser shot to analyze the quantity per single laser pulse. As a rate of the generation per a single pulse, the decreasing profile of the large nanosphere shows an inverse relation to the increasing profile of the network structures. That is, the quantity of network structures increases while that of large nanospheres decreases as the number of laser shot increases. Successive laser irradiation fragmented the large nanospheres and generated the gold nanonetworks. Therefore, the large gold nanospheres are a precursor for the generation of the gold nanonetwork. We measured the extinction spectra of gold nanoparticles in situ. Figure 7 shows the spectra of generated gold nanoparticles, measured at several densities. The spectra contain bands near 530, 650, and 800 nm. The peak near 530 nm has been known to correspond to the plasmon band of nanospheres with diameters
